1. Field of the Invention
The present invention relates to IC preamplifiers, and more particularly, to low noise preamplifiers using capacitive coupling.
2. Relevant Art
In order to read the stored data in a disk drive, a magneto-resistive (MR) head is moved over the surface of the disk. The MR head changes its electrical resistance in response to local variations in magnetic flux. When biased with a constant current, the MR head generates small voltage fluctuations that are representative of the stored data. A differential preamplifier then amplifies the voltage fluctuations to create an output signal that can be used by other components. Because the voltage fluctuations are small, the differential preamplifier must provide low-noise amplification in order to produce an accurate output signal.
Numerous types of differential preamplifiers can be used to generate the output signal. In FIG. 1a, a common base amplifier 170 is shown. In the depicted implementation, an external MR head 120 varies its resistance according a local magnetic flux. A transistor 111 and a transistor 112 form a differential pair, and a transistor 113 and a transistor 114 form a cascode pair. Transistors 111 and 112 are base-coupled by a capacitor 141, while transistors 113 and 114 are connected to a reference voltage Vbb. Biasing for the bases of transistors 111 and 112 is provided by a conventional bias circuit (not shown). A current source 131 provides a bias current for MR head 120 as well as the differential and cascode pairs. Load resistors 115 and 116 convert the bias current into a signal voltage, allowing the voltage across external MR head 120 to be represented at output terminals OUT1 and OUT2. While amplifier 170 provides a relatively simple preamplifier circuit, its gain is limited due to the common base configuration, leading to greater noise susceptibility.
An alternative MR preamplifier 180 shown in FIG. 1b includes transistors 111 and 112 connected as a differential pair. Transistors 113 and 114 are once again a cascode pair. However, current sources 132 and 133 now provide a bias current across MR head 120, creating a voltage differential coupled to the bases of transistors 111 and 112. Current sources 134 and 135 provide biasing for preamplifier 180 in order to produce the appropriate differential output at output terminals OUT1 and OUT2. Because the bias current through MR head 120 typically results in a DC bias of several hundred millivolts, preamplifier 180 must be AC coupled to MR head 120. The AC coupling is provided by an emitter-coupling capacitor connected between the emitters of transistors 111 and 112. However, capacitor 151 introduces a corner frequency fc180, below which the gain of preamplifier 180 begins to roll off. Corner frequency fc180 is defined by the capacitance C151 of capacitor 151 and the emitter resistance Re of transistors 111 and 112. Because the left and right halves of preamplifier 180 are symmetrical, a single ended calculation can be used. Corner frequency fc180 is given by the equation: EQU fc180=1/((Re111*2*C151)*2*pi) [1]
where Re111 is the emitter resistance of transistor 111. A typical MR preamplifier may require a constant gain down to 3.2 Mhz, and a typical emitter resistance will be in the 5 ohm range. Using these numbers, equation 1 indicates that capacitance C151 of capacitor 151 must be: EQU C151=1/(3.2 Mhz*5 ohms*2*2*pi)=5 nF
However, a 5 nF capacitor is too large to integrate into an IC chip, so capacitor 151 must be implemented as an external component. Besides requiring two extra pins in the final chip package, the wiring used to connect to the external capacitor produces parasitic inductances 152 and 153, which prematurely roll off the high frequency response of preamplifier 180.
In order to avoid the problems associated with an external capacitor, base coupling capacitors can be used. As shown in FIG. 1c, a preamplifier 190 is very similar to preamplifier 180. A differential pair made up of transistors 111 and 112 is biased by a current source 134. A cascode pair formed by transistors 113 and 114 drive load resistors 115 and 116, respectively. Base currents for transistors 111 and 112 are provided by resistors 117 and 118, respectively. An external MR head 120 is biased by current sources 132 and 133. However, AC coupling is no longer provided by an emitter-coupling capacitor. Instead, preamplifier 190 includes base coupling capacitors 161 and 162 at the bases of transistors 111 and 112, respectively. A corner frequency fc190 of preamplifier 190 is now given by : EQU fc190=1/((.beta.*Re111*C161)*2*pi)
where .beta. is the current gain of transistor 111, Re111 is the emitter resistance of transistor 111, and C161 is the capacitance of capacitor 161. It should be noted that a single-ended calculation is once again applicable due to the symmetry of preamplifier 190. For a typical transistor current gain of 100, the capacitance C161 required for a 3.2 Mhz low corner frequency is given by: EQU C161=1/(3.2 Mhz*100*5 ohms*2*pi)=100 pF
Although still relatively large, a pair of 100 pF capacitors can be integrated on to an IC chip, eliminating the need for external components. However, although FIG. 1c only shows two amplification transistors (transistors 111 and 112), a typical preamplifier circuit can include many more parallel amplification transistors. While a single emitter-coupled capacitor could serve the multiple amplification transistors, a separate base-coupling capacitor pair would be required for each amplification transistor.
For either type of AC coupling, it is desirable to provide a circuit for reducing the corner frequency of a preamplifier to a desired level without requiring external components or a very large integrated capacitance.